Water absorption by swelling seeds. I. Constant surface boundary condition

Abstract

Recent analyses of water absorption by seeds have used a non-swelling model with constant diffusivity and constant surface concentration boundary condition. A physically realistic modification of such analyses is developed, where the spherical seed has the properties of normal swelling, moisture content dependent diffusively, and a moisture characteristic which is described by the double layer theory relevant to colloids under mechanical restraint. Analysis of this model by a finite difference approximation produces a relationship between linearized moisture content and dimensionless time which is appropriate to all spherical swelling materials. A good match was obtained between this relationship and experimental absorption data for wheat seed. The relationship given graphically in dimensionless form is applicable to any swelling system, e.g. soil aggregates, which satisfy normal swelling and constrained double layer boundary conditions. The analysis gave values for the diffusivity of the seed similar to, but larger than, those calculated from non-swelling models. These are much smaller than most values of diffusivity of soils in the available water range. The magnitude of the maximum rate at which a seed can absorb water relative to that which the soil can supply, predicts that for seeds embedded in most soils, the constant surface concentration boundary condition used by most authors is inappropriate. In terms of the swelling behaviour of most soils, the swelling model, unlike the non-swelling model, predicts that imbibing seed embedded in soil will be subject to a mechanical constraint.